Communications systems which are generally similar to the instant invention are well known to the prior art. Usually the system includes a central or master station which communicates in a regular order with a plurality of remote field stations. The information flow may be over line wires or the signals may be radiated. The master station establishes a regular order of communicating with each of the remote field stations. Remote field stations may be transmitted to sequentially although other orders may be desirable. The information transmitted by the master station comprises orders for the field stations to execute and these orders are termed controls. At the field station, the received control messages are made available to the utilization apparatus through an interface. In turn, on request the field stations will transmit information to the master station respecting the status of the apparatus at the field station. This information is referred to as indications. The general operating sequence is for the master station to establish a regular communication cycle in which it transmits to each of the field stations. Each of the field stations, on reception of a message directed to it, in turn, transmits indications back to the master station.
Applications for systems of the general type referred to above, usually require high security in the messages. That is, the messages, as received, must be substantially error free, and this is especially true for the controls as received by the field stations. In order to assure high reliability of the controls received by the field stations the prior art has established an operating sequence known as select, check, execute. In this sequence, the master station transmits a series of controls to a particular field station. When these controls are received they are transmitted back to the master station, by the field station, as indications. The indications received by the master station are then checked against the controls transmitted by the master station and, if they agree, the master station sends out an additional control message which orders the field station to execute the previously received controls. Although this system does insure high reliability of control messages before they are executed by the field station, those with ordinary skill in the art will understand that the necessity for two transmissions by the master station before a particular set of controls can be executed, reduces the information transmitting capability of the system.
The prior art also illustrates a plurality of types of signals which may be made available to the application circuits, at the field station, through the interface. In one type, the received controls are continually available until such time as another control message is received to cancel a particular control. Alternatively, the system may be arranged so that each of the controls is available only during the period when a message is being received. A system of the foregoing type is described in a manual entitled GRS Code System 4000 Operation and Maintance, Phamplet 1292, revised February 1971. The former type of control is generally referred to as a latch, while the latter type of control is generally referred to as a pulse. Due to the signal requirements of application circuits within a system and even within a particular field station, the inability to selectively supply either pulse or latch outputs is a disadvantage which requires additional complexity in the application circuits. In addition, the pulse type controls available have a predetermined duration, that is the duration of reception of a message. Although the prior art has overcome the disadvantages referred to above, it is only done so with the addition of further complexity in the application circuits.
Many systems are capable of identifying a failure at a particular field station when, for instance, that particular field station fails to respond to transmissions to it, or when a particular field stations transmission is obviously garbled. However, when a particular field station is identified as having failed, the master station may not be capable of exerting any further control over the application circuits located at the particular field station. In some situations it is necessary to effect action at a field station when a communication failure is detected. Obviously the master station may be unable to effect this action.
In some applications it may be desirable to enable control of a particular application circuit, or a group of applications circuits by a particular one of a plurality of control stations. For example, two field stations may occupy the same geographic location; the interfaces of each would enable control of a particular application circuits or a group of application circuits. The two field stations may be controlled by respectively a first and second control office. In the prior art, it has been suggested, that to enable mutual cooperation of the control offices to ensure that one of the control offices exerts control while the other does not, a separate communication path be established between the control offices. The obvious disadvantage to this solution lies in the necessity for an additional communication path which, otherwise would not be necessary. Of course, if a particular output of each of the field stations were tied together the output would be effective if both field stations would achieve the same control for that particular output. This, however, requires both control offices to transmit, and both field stations to receive, the identical control. This redundancy is wasteful of communication time and equipment and does not offer a realistic solution to the problem for a communication path would still be necessary between the control offices to assurre that the controls arrived at the field stations in time synchronism to avoid the possibility of the two outputs being in different states at the same time.